All About Fast-Twitch and Slow-Twitch Muscle Fibers

All muscle fibers aren’t created equal. Muscle fibers, which are cells that contain myofibrils that cause a muscle to contract, come in two main flavors: slow-twitch and fast-twitch. The average person has roughly equal quantities of fast-twitch and slow-twitch muscle fibers, although certain types of athletes typically have a preponderance of one over another.

Each of these types of muscle fiber is ideally suited for a particular type of activity. Slow-twitch muscle fibers, also known as type 1 fibers, is designed for activities that require muscle endurance – walking, jogging, and pedaling leisurely on a bicycle. These fibers use oxidative pathways to produce energy (pathways that require oxygen) and can contract for long periods of time without fatiguing. Without slow-twitch fibers, people couldn’t run marathons.

Fast-twitch muscle fibers, on the other hand, use anaerobic energy pathways, those that don’t require oxygen. These fibers are designed for strength and power moves because they can generate greater force but fatigue more quickly than slow-twitch muscle fibers. Since they use anaerobic pathways to produce energy, lactic acid builds up fairly quickly, the pH, or acid-base balance, drops and you feel fatigued rapidly when you recruit these muscle fibers. Using fast-twitch muscle fibers, you can sprint for short distances, swing a heavy kettlebell, or lift a heavy weight – but you can’t do these activities for very long since these fibers tire easily.

Types of Fast-Twitch Muscle Fibers

Fast-twitch muscle fibers are also referred to as type 2 and there are two sub-types – type 2A and type 2B. It’s the 2B fibers that are “classic” fast-twitch fibers since they have the fastest rate of contraction but also the quickest rate of fatigue. Type 2A fibers are almost like a hybrid of type 1 and type 2B. They are capable of both using both aerobic and anaerobic pathways for energy and are less powerful but also fatigue less quickly than type 2B fast-twitch fibers.

As you might expect, a world class sprinter would likely have a higher proportion of fast-twitch muscle fibers than the average person and an ultra-marathon runner probably has a higher ratio of slow-twitch to fast-twitch fibers. Having a larger number of one type of muscle fiber over the other may offer an advantage in some types of sports but it’s only one factor that determines sports performance.

How do you know what YOUR predominant fiber type is? The only sure way is to get a muscle biopsy. A less accurate method is to see how many repetitions you can do of an exercise using a resistance 80% of your one-rep max. If you can do fewer than seven reps, you probably have greater than 50% fast-twitch fibers. On the other hand, if you can do 12 or more reps, you’re probably slow-twitch dominant.

You might wonder whether you can convert fast-twitch fibers to slow-twitch ones or vice versa through exercise. The answer is yes and no. You can’t take a type 2B fiber and turn it into a type 1, slow-twitch fiber through training, BUT type 2A fibers, those that are intermediate in terms of strength and endurance, can take on the characteristics of one fiber type or the other through intensive training.

Don’t forget how strong and powerful you are also has to do with the effect your nervous system has on fast-twitch fibers. Through strength and power training, you can train your nervous system to maximally activate the fast-twitch fibers you have. What type of training would you use? Heavy resistance training, ballistic movements, and plyometrics, movements that force your muscle fibers to adapt and become stronger and more powerful are ideal.

How does muscle fiber recruitment take place? When you lift, you initially call slow-twitch fibers into play. If the weight is light enough, there’s no need to recruit fast-twitch muscle fibers. It’s only when you use a heavier resistance that fast-twitch fibers get in on the action. Ideally, for hypertrophy, you want to lift at around 80% of your rep max for 8 to 10 reps.

Why Strength and Power Training is So Important

You may have roughly equal fast-twitch and slow-twitch fibers when you’re 30, but you lose fast-twitch fibers faster as you age than slow-twitch ones. By age 80, you may have lost almost half of your fast-twitch muscle fibers. That’s one reason elderly people who are deconditioned have problems getting up out of a chair. They don’t have enough thigh and hip strength to lift themselves up.

Exercise helps to offset this decline in fast-twitch muscle fibers, according to some research, but it must be strenuous exercise. Lifting three-pound dumbbells multiple times isn’t an intense enough stimulus to call fast-twitch muscle fibers into action. With lighter weights, you’re recruiting mostly slow-twitch muscle fibers. You’re enhancing muscle endurance but doing little to build strength and power OR preserve fast-twitch muscle fibers.

The tendency, as we get older, is to lighten up on the weights and do exercises that target slow-twitch fibers like walking and leave our fast-twitch muscles to languish and die a slow death. What you don’t use, you lose. No doubt, training that targets fast-twitch muscle fibers carries a higher risk of injury and some older people have orthopedic issues that make it harder to do certain exercises. They key is to choose appropriate exercises and allow enough recovery time between higher intensity workouts. As you age, you may need to reduce the number of days per week you train your fast-twitch fibers to give your muscles more recovery time – but you still need to train with intensity to retain those fast-twitch muscle fibers.

The Bottom Line

Love and respect your fast-twitch muscle fibers – and keep them in shape by training them regularly. They’re the ones you’ll lose the most of as you age unless you do. However, make sure you’re giving your muscles adequate recovery time after a high-intensity exercise session.